A local area network (LAN) refers to a group of networked computers or nodes, such as personal computers or workstations, connected by a communications link that enables the network nodes to share information and resources within a relatively limited area. A token ring network is a local area network formed in a ring or closed loop topology that uses token passing to communicate between nodes on the network.
An exemplary token ring, local area network is illustrated in FIG. 1. The network 100 includes five network nodes, referenced individually as 101, 102, 103, 104, and 105. These five network nodes are coupled through respective coupling units, referenced individually as 111, 112, 113, 114, and 115 to a ring bus 120. Although five nodes are shown in FIG. 1, network 100 can contain many more than the five nodes shown.
To orderly manage accesses to bus 120, a token frame, or simply a token, is passed around ring bus 120 from one network node to another. The process for transmitting information from a first, or sending, network node to a second, or receiving, node along the ring bus is accomplished as follows. The sending node waits for a token, seizes the token, marks it as being in use, inserts the information to be transmitted into a message frame appended to the token, and releases the token to the ring bus for transmission to the next node. The token may only be held by one node at any one time. The message frame includes the address of the receiving network node to which the message is directed. The message frame is passed from node to node along the ring to the receiving node and continues until it is eventually transmitted back to the sending network node. While the message frame is being passed around, the intended receiving network node retains a copy of the message frame and indicates that it has done so by setting a predetermined field called a frame-copied field in the message frame. If the message frame is not successfully copied, as indicated by the state of the frame-copied field, the sending network node will re-send the token and message frame.
Each of the five network nodes shown in FIG. 1 includes a network interface including a medium access control (MAC) unit, an elastic buffer, and protocol control software. The MAC unit is responsible for performing frame encapsulation and de-encapsulation. In many encoding schemes, such as the commonly used Manchester encoding scheme, a reference clock is encoded into the binary data bit stream prior to transmission. The encoded data stream generated in accordance with the Manchester encoding scheme has a transition (positive or negative) in the middle of each bit cell period with the effect that the reference clock is readily extracted from the received encoded bit stream. The elastic buffer is responsible for synchronizing the reference clock with the bit cells in the data stream.
The protocol control software includes a monitor program. Among the five network nodes shown in FIG. 1, the monitor program is only in an active state in one node at any given time. The network node having the active monitor is referred to as an active network node, while the remaining network nodes are referred to as participating network nodes. The active monitor provides a reference clock to which all participating network nodes are synchronized for encoding or decoding the Manchester encoded signals.
While a token and included data is being passed from node to node along the ring bus, it can be distorted and data lost as a result of impedance mismatching of the transmission wires that form the ring bus. Such distortion can cause errors in phase alignments between bit cells and the reference clock, which in turn can result in encoding and decoding errors of the data stream. To recover the data stream, an elastic buffer within the active network node must realign the incoming signal with the reference clock. The active monitor in the active network node provides the realigned signal to all other participating network nodes. However, if the distortion is too great to be corrected, the active monitor will transmit a MAC error frame to denote the problem.
In the token ring environment, a LAN network is in a marginal condition when it can operate smoothly in idle or light traffic situation, but exhibits severe performance problem when the network is stressed with heavy traffic. Frequently, the marginal condition is not problematic until physical length is added to the LAN, i.e., additional length of transmission line is added to the network ring bus. Each component on a transmission line (wire, connectors, filters, multistation access units, etc.) has the potential to introduce an impedance mismatch that can result in a phase distortion. As components are added to the network, tolerances are approached or exceeded, resulting in transmission problems.
Because the symptoms of a marginal LAN network appear to be random, it is difficult to locate the root cause of the problem. Conventionally, a token ring network has to be shut down to locate the problem causing the marginal condition.
Therefore, there exists a need for a method to efficiently identify problems causing marginal condition in a token ring network, desirably without requiring that the token ring network be shut down to identify the problem affecting the performance of the network.